Sci Eng Compos Mater 2017; 24(2): 195–202 Aiguo Liu*, Da Li, Fanling Meng and Huanhuan Sun Metal-matrix composite fabricated with gas tungsten arc melt injection and precoated with NiCrBSi alloy to increase the volume fraction of WC particles DOI 10.1515/secm-2014-0221 investigation of Liyanage et al. [1] on plasma transferred Received July 12, 2014; accepted June 1, 2015; previously published arc welded Ni-WC overlays shows that the MMC overlays online September 5, 2015 are from two to five times more wear resistant than the matrix alloys without WC particles. Abstract: The volume fraction, dissolution, and segrega- Many welding and cladding methods have been used tion of WC particles in metal-matrix composites (MMCs) to produce WC particle-reinforced MMCs on low-cost are critical to their wear resistance. Low carbon steel substrates. However, there are still some problems to be substrates were precoated with NiCrBSi coatings and solved. The first problem is the degree of dissolution of processed with gas tungsten arc melt injection method to WC particles. In the process of welding or cladding, WC fabricate MMCs with high volume fraction of WC particles. particles are heated by a heat source and dissolve into The microstructures and wear resistance of the compos- the molten pool partially or even completely. Katsich and ites were investigated. The results showed that the volume Badisch [2] have accessed the effect of carbide degrada- fraction of WC particles increased with decreasing hopper tion in a WC/W C-reinforced Ni-based hard facing, and the height and was as high as 44% when hopper height was 2 results show significant carbide degradation with increas- 100 mm. The dissolution of WC particles was minimal. The ing welding current, resulting in a significant reduced content of the alloying elements decreased from the top to primary carbide content and carbide diameter. Reduced the bottom of the matrix. More WC particles dissolved in carbide content indicated a significant wear rate increase the overlapping area, where Fe W C carbide blocks could 3 3 under pure three-body abrasion conditions. Dissolution be found. The wear loss of the MMCs after 40 min was results in lowered wear resistance, as there are less WC 6.9 mg, which is 76 times less than that of the substrate particles that remained to provide wear protection. The after the 4 min test. contents of carbon and tungsten in the matrix increase and the toughness of the matrix decreases, so the risk of Keywords: gas tungsten arc melt injection; metal-matrix cracking increases. The extra carbon and tungsten precip- composite; NiCrBSi alloy coating; volume fraction; WC itate as M C-type carbides, which are less wear resistant particle. 6 than WC particles. The second problem is the volume fraction of WC particles in the matrix. Jankauskas et al. [3] have inves- 1 Introduction tigated the effect of WC grain size and content on low stress abrasive wear. They have found that the wear rate The wear resistance of metal-matrix composites (MMCs) of hard facing decreases with the increase in WC content, with remarkable content of hard phases with very and a factor of 9 has been achieved when WC content is high hardness, such as carbides, is well known. The 42–43 wt%. A further increase in WC content is generally considered difficult because of the problem of WC particle dissolution. *Corresponding author: Aiguo Liu, School of Materials Science and The third problem is the distribution of WC particles Engineering, Shenyang Ligong University, Shenyang 110159, in the matrix. Fernández et al. [4] have studied the tribo- P.R. China, e-mail: [email protected] logical improvement of NiCrBSi laser cladding coating Fanling Meng and Huanhuan Sun: School of Materials Science and Engineering, Shenyang Ligong University, Shenyang 110159, reinforced with different weight percentages of WC parti- P.R. China cles. They have found that WC particles tend to precipitate Da Li: Cixi Guanghua Industrial Co. Ltd., Cixi 315326, P.R. China at the bottom of the melt coating; hence, the percentage 196 A. Liu et al.: Gas tungsten arc melt injection of WC particles of carbides increases with the depth of the coating. The to lower the surface tension, to allow more WC particles to density of WC particles is much higher than that of the be incorporated into the MMCs. The microstructures and matrix. They are prone to sink down at the bottom of wear resistance of the MMCs were reported. the molten pool. The tendency of segregation of WC par- ticles is related to the time they spend in the molten pool, which is determined by the procedure and process para- meters. With a certain procedure and fixed parameters, little 2 Materials and methods can be done to control the distribution of WC particles in the matrix. The segregation of WC particles decreases the The substrate was of Q235 low carbon steel (similar to ASTM wear resistance of the top layer of the MMC and increases A570 Gr.A; Anshan Steel Co., Ltd., Anshan, China). The the cracking tendency of the MMC/substrate interface. chemical composition and mechanical properties of the To solve the problem of dissolution and segregation substrate are shown in Table 1. The dimensions of the sub- of WC particles, Vreeling et al. [5] proposed a laser melt strate were 250 × 50 × 5 mm3. The NiCrBSi alloy powder used injection process. In the laser melt injection process, a for precoating was manufactured by VEHA Co., Ltd. (Keo- laser beam is used to melt the substrate to form a molten rekovsk, Russia), a Russian company that manufactures pool. With the movement of the laser beam, a tail is thermal spray equipment and materials. The morphology formed behind the molten pool, and the carbide parti- of the NiCrBSi powder particles is shown in Figure 1. The cles are injected into the tail of the molten pool, avoiding nominal diameters of the particles were 5–45 μm. The com- direct interaction with the laser beam. With this method, position of the NiCrBSi powder is shown in Table 2. The they have strengthened titanium alloy with WC particles injected particles were cast and crushed WC-8% Co parti- [5] and aluminum alloy with SiC particles [6]. Zhao et al. cles (Harbin Welding Institute, Harbin, China). The size of [7] use a plasma arc to melt and inject WC particles into the WC-8% Co particles was 350–700 μm. low carbon steel substrates. Other carbide particles, such The substrate was degreased, dried, and grit blasted. as Cr3C2-NiCr [8] and SiC [9], have also been successfully The NiCrBSi powder was flame sprayed onto the substrate. incorporated into overlays with little dissolution by the The coating was 1 mm thick. melt and injection processes. The schematic diagram of the GTAMI system is shown However, the volume fraction of the incorporated in Figure 2. The coated substrate was fixed on a movable carbide particles is still limited. It is found that only the platform. The moving velocity of the platform was adjust- particles with velocity higher than a critical velocity vc can able. A gas tungsten arc torch was set above the platform overcome the surface tension and enter the molten pool with an angle α to the normal of the platform. The sub- [6]. To increase the volume fraction of the carbide particles strate with coating was heated by the arc, and a molten in the MMCs, the velocity of the injected particles must be pool formed under the arc. The platform moved horizon- increased or the surface tension of the molten pool must tally with a velocity of 4 mm/s. The molten pool left a tail be decreased. NiCrBSi alloys, which have low melting behind the arc. The injection nozzle was fixed together point, low surface tension when melted, and excellent with the torch with an angle β to the normal of the plat- wear resistance, are often sprayed or cladded on indus- form. WC particles were stored in a hopper. The height trial components for wear protection [10]. If they are pre- of the hopper was adjustable. WC particles were driven coated on the substrate, the surface tension of the molten out of the hopper by a roller and were accelerated down pool will be greatly decreased. Therefore, the volume frac- through the pipe and out the injection nozzle by gravity. tion of WC particles in the MMCs can be increased. WC particles left the nozzle with a certain velocity, entered In this paper, WC particle-reinforced MMCs were fab- the tail of the molten pool, and were caught in the matrix ricated on Q235 low carbon steel substrates with the gas after the solidification of the molten pool. The parameters tungsten arc melt injection (GTAMI) process. NiCrBSi alloy of the GTAMI process are shown in Table 3. Single-pass was deposited on the substrate before melt and injection, specimens with different hopper heights and multipass Table 1: Chemical composition and mechanical properties of Q235 steel. Chemical composition (wt%) Mechanical properties C Si Mn P S Fe σs (MPa) σb (MPa) Elongation (%) 0.14–0.22 0.30 0.3–0.65 ≤ 0.045 ≤ 0.05 Bal. 235 375–500 26 A. Liu et al.: Gas tungsten arc melt injection of WC particles 197 (5 g FeCl3, 25 ml HCl, 25 ml ethanol). The microstructures of the MMCs were investigated with a Hitachi model S-570 scanning electron microscopy (SEM). The compositions of different phases were analyzed with a Tracor North- ern model TN-5502 energy-dispersive X-ray spectroscopy (EDS). The phases in the MMCs were analyzed with a model D/max-rB X-ray diffraction (XRD) analyzer (Rigaku, Japan).